(1) Field of the Invention
The present invention relates to a high-performance electret which can stably hold a high surface charge density over a long period and can be formed into a film. Furthermore, the present invention relates to an air filter having excellent efficiency in dust collection, which is fabricated by using this electret.
(2) Description of the Prior Art
Various proposals have been made on polymeric electrets. Most of the known polymeric electrets are composed solely of a non-polar or polar polymeric material or binary electrets comprising a non-polar polymer and a polar polymer. Characteristics of these known polymeric electrets will now be described. In the case of an electret of a non-polar polymer represented by polyethylene or polypropylene, since the electric conductivity is low, charges once trapped hardly disappear, and since the polymer is hydrophobic, it is said that charges hardly disappear even if the electret comes in contact with water. Furthermore, since the polymer has no polarity, the quantity of charges trapped at the electret-forming step is small and the capacity of the formed electret is insufficient. In the case of a polar polymer such as polyethylene terephthalate or polycarbonate, since the polymer contains a polar group in the molecule unlike the above-mentioned non-polar polymer, the quantity of charges trapped at the electret-forming step is large, and the formed electret is characterized in that the initial electret capacity is high. However, since the electric conductivity is high, charges readily disappear with the lapse of time and it is difficult to maintain a high electret capacity for a long time, and since the polymer is hydrophilic, charges readily disappear if the electret comes in contact with water. Binary electrets comprising these two types of polymers have been developed and proposed so as to utilize the merits of both the polar and non-polar polymers and provide electrets retaining the excellent characteristics of both the polymers. In connection with these binary electrets, it is known that a blend system in which a non-polar polymer is a matrix and a polar polymer is a domain is excellent as an electret over a blend system of a reverse structure in which a polar polymer is a matrix and a non-polar polymer is a domain. For example, in "Collection of Lectures on Polymers", Volume 38, No. 9 (1981), pages 587-591, it is reported that an electret can be formed from a binary blend comprising polystyrene as the non-polar polymer and chlorinated polyethylene as the polar polymer. It is taught in this report that in the case of a multiple-phase blend, charges are easily trapped in the boundary region between components and that in the case of a micro-phase dispersed structure in which polystyrene is a domain and chlorinated polyethylene is a matrix, charges trapped in the polystyrene/chlorinated polyethylene interface are readily removed, migrate through the continuous matrix of chlorinated polyethylene having a low electric resistance and disappear. Also it is taught that in the case of a micro-phase dispersed structure in which polystyrene is a matrix and chlorinated polyethylene is a domain, charges trapped in the interface are inhibited from migrating by the polystyrene matrix which is an insulating phase and damping of charges is hence delayed. However, the compatibility between these non-polar and polar polymers is poor and therefore, according to the ordinary melt-blending method, it is impossible to reduce the particle size of the domain to several .mu.m. This means that the ordinary melt-blending method can hardly be adopted for obtaining a filmy electret having a thickness of several .mu.m, rather a special blending method such as the solvent method should be adopted. However, adoption of the solvent method is not preferred from the industrial view point because the electric characteristics are substantially influenced by the residual solvent and complicated steps are necessary for evaporation and recovery of the solvent.